#include<cstdio>
#include<cstring>
int a,b,x,y,n,m,k,c,st[11],q[11],nr,i;
int main ()
{
freopen("cifre.in","r",stdin);
freopen("cifre.out","w",stdout);
scanf("%d%d%d%d",&a,&b,&k,&c);
for(i=a;i<=b;i++)
    {
    int qq=0,c1=i;
    while(c1!=0)
        {
        if(c1%10==c)
            qq++;
        c1/=10;
        }
    if(qq>=k)
        nr++;
    }
printf("%.4lf",1.0*nr/(b-a+1));
return 0;
}